Numerous techniques and devices are known and available for imaging structures within opaque or turbid objects, such as biological tissue. Various examples of such prior art are provided in my copending U.S. application, Ser. No. 07/722,823, filed Jun. 28, 1991, the contents of which are hereby incorporated by reference.
The copending application describes a method and apparatus for imaging turbid media using optical interference among diffusively propagating, phase encoded, intensity modulated optical carriers, utilizing photons which pass through the medium (such as biological tissue) by the wave diffusion process as opposed to geometrically propagating "prompt" photons. As disclosed therein, a method for imaging an object includes the steps of applying modulated optical rays at a plurality of points along a surface of the object, each of the rays characterized by an amplitude modulated at a respective modulating frequency and by a specific phase. The phases applied to the various rays are provided respective relative phase-shifts which are selected to cause constructive interference among the modulated rays at a predetermined volume element of interest, or voxel.
Thus, the method effectively selects the predetermined voxel for imaging by selecting the relative phase-shifts to be applied to the intensity modulated light rays. In accordance with the method, the modulated rays are diffusively propagated through the object and the intensity and phase of a light ray resulting from the constructive interference at the selected voxel are detected in order to image a characteristic of the voxel.
In order to image a portion of the object which includes a number of voxels, the method of the copending application may include the further steps of repeating the selecting step for a sequence of predetermined voxels, repeating the propagating step to diffusively propagate the modulated rays through the object to the predetermined voxels, and repeating the detecting step to detect rays respectively resulting from a sequence of constructive interferences at the sequence of voxels. For such a process, the selecting step preferably includes the steps of using a signal responsive phase shifting device, such as a zone plate, for applying the relative phase-shifts to the rays. Computer generated signals are applied to the signal responsive phase shift device, thereby providing non-mechanical scanning of the portion of the object to be imaged.
Although the above summarized invention disclosed in my copending application solves a number of problems associated with the prior art, the disclosed method and apparatus provides for imaging of absorption characteristics using interference among diffusively propagating modulated light rays. There accordingly remains a need for method and apparatus capable of imaging attenuation and phase delay characteristics of an object, using a simplified detector structure and not necessarily relying on occurrence of an optical interference at the object.
In that regard, the prior art uses an optical mask which is time encoded to permit various light rays to impinge on specific voxels of an object. After passing through the object, the rays emerge with intensity attenuation and phase change determined by the characteristics of the voxels. The emergent rays are directed to be incident on corresponding photodetectors for imaging thereby. Such a prior art arrangement is illustrated in FIG. 8, wherein a n plurality of intensity modulated light rays r.sub.i are transmitted through corresponding voxels V.sub.i (i=1, 2, 3, . . . , 4) of an object 10 for detection by a photodetector array 13. A mask 11 includes a plurality of opaque and transparent regions in the paths of light rays r.sub.i to transmit or block specific rays to the object and to establish a specific timing sequence of rays for incidence on the object.
Such a mask may be a computer controlled liquid crystal plate, for example, having pixels or pixel groups which are selectively made opaque and transparent according to a predetermined timing sequence. In the Figure there is illustrated a single transparent region 15 in mask 11, the remaining regions being opaque. By having a predetermined time sequencing for the incident rays as well as for reading and processing of outputs of the photo-detectors 13, a processor 16 obtains a distribution of the intensity (amplitude) attenuation and phase shifting characteristics of the various voxels of the object, thus imaging the object.
However, such a prior art approach as shown in FIG. 8 requires a complex sequencing arrangement of the imaging rays, requires a complex photodetecting structure utilizing a number of photo-detectors, and requires passage of time from the application of the first imaging ray to the last such ray, thus effectively prohibiting the system from obtaining a real-time "snapshot" of the voxels of the object.
There is thus a need in the prior art for method and apparatus of using a single detector cell rather than an array of cells for imaging attenuation and phase delay characteristics of one or more elements which are arrayed along a plurality of dimensions of an object, whether the imaging uses a single exposure or a plurality of exposures.
There is yet a more particular need for a method and apparatus for imaging a plurality of voxels of an object of interest, whether disposed as a one-dimensional linear array, as a two-dimensional planar array, or as a three-dimensional volume of the object, by a single exposure of the array of voxels to imaging light including a plurality of phase-encoded, modulated optical carrier frequencies.
There is yet another need in the prior art for imaging one or more voxels of an object by applying one or more rays of light to the voxels, in one or more exposures, using geometrically propagating light rays.